The third book of the Compendium opens with several chapters on the anatomy and physiology of the eye and the phenomena of vision. According to Gilbert, the eye consists of three humors, the albugineous (aqueous), the crystalline lens and the vitreous humor, and seven tunics, apparently
1. The conjunctiva
2. The albuginea or sclerotic
3. The cornea
4. The secundina (choroid)
5. The rethilea (retina)
6. The aranea (iris)
7. The uvea perforata (posterior layer of iris),
though the definitions are not in all cases quite clear and definite. The tela aranea is said to take its origin from the retina, the retina from the optic nerve, and the latter from the rethi (rete, network) involving the substance of the brain. The cornea arises from the sclerotic tunic, the uvea and secundina take their origin from the pia mater, and the conjunctiva from a thin pellicle or membrane which covers the exterior of the cranium and is nourished by a transudation of the blood through the coronal suture. This pellicle is also said to have a connection with the heart, which arrangement furnishes a decidedly curious explanation of the mechanism of sympathetic and maudlin lachrymation. For, as Gilbert tells us, when the heart is compressed this pellicle is also compressed, and if any moisture is found beneath the pellicle it is expressed into the substance of the lachrymal gland by the constriction of the heart, and men in sorrow therefore shed tears. And again, if the heart is much dilated or elevated (by joy), this pellicle is also dilated or elevated, and if any moisture is found beneath it, it is expressed in the form of tears. Accordingly, men who are too joyful shed tears. Still further, drunken men, who are notoriously "moist," and have a superfluity of fluid between the pellicle and the skin of the cranium, are prone to weeping on slight provocation, and their tears are nothing more than an expression of this moisture, which makes its exit, not through the substance of the eye, but through the "lachrymal angle." Q.E.D.
This odd demonstration is followed by a succession of optical questions, which are discussed and answered in true scholastic style, with no little acuteness of observation. Thus: "Utrum visus fiat intus suscipiendo?" Is vision accomplished by something received into the eye? "Utrum color fit de nocte?" Does color exist at night? To the latter question Gilbert replies that in the darkness color exists in posse, but not in esse. Again: "Why do some animals see at night, some in the day only and some only in the twilight?" This phenomenon he ascribes to "the clearness and subtilty of the visual spirits, or to the strength, weakness, grossness or turbidity of the organs of vision." Some animals, he says, have (visual) spirits, subtle and clear as fire, and these animals see perfectly at night because the visual spirits (spiritus visibilis) are sufficient to illuminate the external air. "Why do objects in water seem nearer than those in air?" Gilbert explains this as follows: "Nothing appears distant, except as perceived through an extensive intervening medium. But our judgment is largely guided by the transparency of this medium, since the medium itself is not perceived with much accuracy, except when it is transparent. Accordingly, as the lucidity of air is greater than that of water, an object looks more distant through air than through water."